1. Field of the Invention
The present invention is directed to apparatus and method for drilling a well into a submerged bottom from a floating drilling vessel with a riser having positive buoyancy.
2. Description of the Prior Art
An increasing amount of drilling is being conducted offshore in an attempt to locate new oil fields. In deep water, exploratory wells are generally drilled from floating vessels. As in onshore operations, drilling fluid must be circulated through the drill bit to keep it cool and carry away drill cuttings. Normally, the drilling fluid is returned from the well to the water surface by means of a large diameter pipe, known as a riser, extending between the vessel and the subsea well. The drill string extends through the riser, and drilling fluid circulates downwardly through the drill string, out through the bit and upwardly through the annular space between the riser and drill string.
As the water depth in which drilling operations are conducted increases, the static weight of the riser becomes so great that it will tend to buckle under its own weight unless supported. In deeper water, tensile force in excess of that required merely to support the weight of the riser must be applied to the riser to prevent forces imparted by waves, currents, and dense drilling fluids from distorting or even buckling the riser pipe. This axial tensile force is normally applied to the riser by means of tensioning devices situated on board the vessel. In very deep water, however, it becomes impractical to provide all of the supporting force from the drilling vessel because of the size and cumbersomeness of the tensioning devices required.
Since the magnitude of tensile force that can practically be developed on the vessel is limited, in very deep water it would be desirable to add enough buoyant material to impart positive buoyancy to the riser. Synthetic foam and other buoyant materials have been attached along the length of the riser for this purpose, but this technique presents problems of its own.
One problem is that, in the event of accidental serverance of the riser or its release from the underwater wellhead, the buoyancy is apt to propel the riser upwardly out of the water, creating the likelihood of damage to property and injury of personnel. Another problem with a positively buoyant riser is that it complicates the procedure required for temporary abandonment of the drilling location if inclement weather forces the drilling vessel to leave the area, making it difficult to move off the drilling site in a short period of time. Accordingly, while a positively buoyant riser offers a number of advantages, the problems accompanying such sytsems have limited there use in the past.